In integrated circuit design, great emphasis is placed on the operating speed of a circuit. In fixed-memory devices, such as programmable array logic (PAL), a current source is often coupled to each product line of the
in order to decrease the product line high-to-low transition time.
In operation, the current sources remain on continuously. When the inputs to all of the emitter follower transistors connected to a product line are low, it is desired that the logic level on the product line be low. The current source associated with the product line "pulls down" the emitter followers, turning them off more rapidly than they would without the current source. The result is that the product line also pulls down more rapidly.
When one of the inputs is high, it is desired that the logic level on the product line also be high. In order for the emitter follower connected to the high input to turn on, it must overcome the pull down from the current source. This requires more than would be needed without the current source.
Thus, while this circuitry is faster than a PAL without current sources, it has the disadvantage of drastically increasing the power consumed by the device. A further disadvantage is that increasing the current to increase the high-to-low transition speed also decreases the low-to-high transition speed.
Therefore, a need has arisen to provide a circuit for providing a fast high-to-low product line transition speed without increasing the low-to-high transition speed and further, without increasing power consumption of the device.